Senior Member

If the fire alarm goes off, it means it starts making noise. This is already completely clear. If there isn't actually a fire, it will often then need to be manually turned off. If it stops by itself, we might say that it has 'turned itself off' or simply 'stopped'.

Senior Member

There are two problems:
1. No one says, "Fire alarm goes off", they say "The fire alarm goes off." or "A fire alarm goes off.", etc.
2. There is always context, and this will make "to go off" clear.
3. "To go off" is a phrasal verb. Usually, there is a clearer non-phrasal verb:

It depends on the noise the particular alarm makes. Some do not have a “ringing” sound: they may buzz, or beep, or go off like a siren or klaxon horn. In those cases, “has been sounding” would be more accurate.

I have to wonder, though, how a fire alarm could go off for such a long period and not be reported. Are you sure you’re not talking about a smoke detector in a single unit, rather than a fire alarm for an entire building?